16.8 Aspergillosis disease of coral
Decline of coral reef ecosystems has been a concern since the 1970s. Overall, the loss of coral reefs is thought to be a combined result of global warming, ozone depletion, overfishing, eutrophication, poor land-use practices and other expressions of human activities. There are, however, reports that emphasise the role of coral disease in reef degradation. Many aspects of coral disease are poorly understood, but disease syndromes do seem to be increasing, and may parallel a general decline in the marine environment. However, much of the data is purely descriptive and though viruses, bacteria and fungi have all been implicated as disease organisms most reports lack proof that the alleged pathogen is indeed the cause of the disease (Richardson, 1998; Harvell et al., 1999; Work et al., 2008).
One disease for which the identity of the pathogen has been proved to a satisfactory level is aspergillosis of sea fans. Sea fans (Gorgonia ventalina) suffered mass mortality on reefs in the Caribbean and the Florida Keys in 1995 and 1996. Symptoms of the disease included expanding areas of the sea fan in which polyp tissues were destroyed, exposing the axial skeleton beneath. In some cases the lesions were so severe that holes appeared in the skeleton. The disease was common over a wide geographical area and though partial destruction of individual Gorgonia colonies was most usual, the entire fan was sometimes destroyed. It was demonstrated that tissue-degrading lesions all contained fungal hyphae at their edges and careful analysis demonstrated that:
- the same filamentous Aspergillus species was isolated from different geographical disease areas,
- the organism could be grown in pure culture in vitro,
- mycelium taken from culture and inoculated onto healthy sea fans caused typical disease symptoms,
- the same fungus could be recovered from these experimental disease instances and recultured in vitro (Smith et al., 1996).
The course of action followed here was established by Robert Koch in the 1870s to identity pathogenic microorganisms (they are called Koch’s postulates). Such procedures are necessary to demonstrate unambiguously that a presumed disease pathogen is the true cause of a disease (see discussions in Richardson (1998) and Work et al. (2008). This initial study identified the fungus as Aspergillus fumigatus, but it was later shown to be A. sydowii (Geiser et al., 1998). The disease still persists in the western Atlantic and was probably the cause of mass mortality in sea fans that occurred throughout the Caribbean during the 1980s (Richardson, 1998). A. sydowii is a common saprotroph that is found in both terrestrial and marine environments. The species is widespread, having been isolated from terrestrial environments as varied as Arctic soils in Alaska and soils in tropical regions, as well as from subtropical marine waters in both coastal regions and oceanic zones as deep as 4,450 m. Although A. sydowii is common and cosmopolitan, it had not previously been recognised as causing disease in plants or animals. However, nonmarine strains of A. sydowii did not cause disease in sea fans so it was concluded that isolates taken from diseased corals have acquired pathogenic potential not seen in isolates from other sources.
Several Aspergillus species are known to cause opportunistic infections of animals; generally attacking individuals with weakened immune systems or other defences. The infection of sea fans by A. sydowii may also be opportunistic, the fungus taking advantage of a host weakened by pollution or other environmental factors. Also, disease-causing isolates may have a specific pathogenicity factor, perhaps a mycotoxin or other secondary metabolite, that is not present in all isolates. Emergence of A. sydowii as a marine pathogen implies that the land-sea boundary is not an effective barrier to disease transmission (Harvell et al., 1999).
Studies on amphibian chytridiomycosis and coral aspergillosis, particularly concerning the origin of the epidemics and how common saprotrophic fungi gained the ability to cause disease in animals with which they have co-existed for millions of years, will help us understand how new diseases emerge. This is not only important for amphibian and cnidarian populations, but for ourselves, too. We may be next in line.
Updated December 17, 2016